Rename runAsync to scheduleMicrotask.

tests/lib/async/schedule_microtask6_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library run_async_test;
 
 import 'dart:async';
 import '../../../pkg/unittest/lib/unittest.dart';
 
 bool _unhandledExceptionCallback(exception) => true;
 
 main() {
   test('run async test', () {
     var callback = expectAsync0(() {});
-    runAsync(() {
-      runAsync(() {
+    scheduleMicrotask(() {
+      scheduleMicrotask(() {
         callback();
       });
       throw new Exception('exception');
     });
   });
 }